U.S. patent number 5,858,462 [Application Number 08/676,289] was granted by the patent office on 1999-01-12 for porous metal-oxide thin film and method of forming same on glass substrate.
This patent grant is currently assigned to Central Glass Company, Limited. Invention is credited to Seiji Yamazaki.
United States Patent |
5,858,462 |
Yamazaki |
January 12, 1999 |
Porous metal-oxide thin film and method of forming same on glass
substrate
Abstract
The invention relates to a porous metal-oxide film formed on a
glass substrate by the sol-gel process. The film is prepared by a
method comprising the steps of: (a) preparing a coating solution by
mixing a metal alkoxide(s) and/or a metal acetylacetonate(s), a
first solvent, water, an acid and an organic polymer; (b) applying
the coating solution to the glass substrate; (c) drying a gel film
formed thereon at a first temperature not higher than 100.degree.
C.; (d) contacting the gel film with a second solvent which is one
of an acid solution and a mixed solution of alcohol and water; and
(e) heating the gel film at a second temperature so as to transform
the gel film into the porous metal-oxide film. The porous
metal-oxide film's surface is minutely rough and has a fine
features pattern of three-dimensional micro-porous structure.
Inventors: |
Yamazaki; Seiji (Matsusaka,
JP) |
Assignee: |
Central Glass Company, Limited
(Yamaguchi, JP)
|
Family
ID: |
22249769 |
Appl.
No.: |
08/676,289 |
Filed: |
July 18, 1996 |
PCT
Filed: |
August 14, 1995 |
PCT No.: |
PCT/US95/11512 |
371
Date: |
July 18, 1996 |
102(e)
Date: |
July 18, 1996 |
PCT
Pub. No.: |
WO97/06896 |
PCT
Pub. Date: |
February 27, 1997 |
Current U.S.
Class: |
427/226; 427/380;
427/389.7 |
Current CPC
Class: |
C03C
17/007 (20130101); C03C 17/25 (20130101); C03C
2217/425 (20130101); C03C 2218/32 (20130101); C03C
2218/113 (20130101); C03C 2217/22 (20130101); C03C
2218/328 (20130101); C03C 2217/213 (20130101); C03C
2217/212 (20130101); C03C 2217/77 (20130101) |
Current International
Class: |
C03C
17/25 (20060101); B05D 003/02 () |
Field of
Search: |
;427/226,389.7,380 |
References Cited
[Referenced By]
U.S. Patent Documents
Primary Examiner: Bell; Janyce
Attorney, Agent or Firm: Evenson, McKeown, Edwards &
Lenahan P.L.L.C.
Claims
What is claimed is:
1. A method of forming on a glass substrate a porous metal-oxide
film, the method comprising the steps of:
(a) mixing together at least one compound selected from the group
consisting of metal alkoxides and metal acetylacetonates, a first
solvent, water, an acid and an organic polymer, so that a
hydrolysis and polycondensation of said at least one compound
occurs in the presence of said organic polymer and thus that a
coating solution in the form of sol having a single phase is
prepared;
(b) applying said coating solution to said glass substrate, so that
a gel film is formed on said glass substrate, said gel film having
inorganic and organic-polymer phases into which said single phase
has been separated due to an evaporation of said first solvent;
(c) drying said gel film at a first temperature so as to
substantially completely evaporate said first solvent;
(d) removing said organic polymer phase from said gel film, by
contacting said gel film with a second solvent which is one of an
acid solution and a mixed solution of alcohol and water; and
(e) heating said gel film at a second temperature so as to
thermally decompose said organic-polymer phase still remained in
said gel film and so as to transform said gel film into said porous
metal-oxide film.
2. A method of forming on a glass substrate a porous metal-oxide
film, the method comprising the steps of:
(a) preparing a coating solution which is in the form of sol and
has a solid matter concentration within a range from 0.01-0.50 wt %
based on the total weight of said coating solution, by mixing
together at least one compound selected from the group consisting
of metal alkoxides and metal acetylacetonates, a first solvent,
4-10 mols of water to 1 mol of said at least one compound, an acid
in an amount to adjust said coating solution to a pH within a range
from 1 to 3, and 5-30 wt %, based on the total weight of said
coating solution, of an organic polymer, so that a hydrolysis and
polycondensation of said at least one compound occurs in the
presence of said organic polymer;
(b) applying said coating solution to said glass substrate, so that
a gel film is formed on said glass substrate, said gel film having
inorganic and organic-polymer phases into which said single phase
has been separated due to an evaporation of said first solvent;
(c) drying said gel film at a first temperature not higher than
100.degree. C., so as to substantially completely evaporate said
first solvent;
(d) removing said organic-polymer phase from said gel film, by
contacting said gel film with a second solvent which is one of an
acid and a mixed solution of alcohol and water; and
(e) heating said gel film at a second temperature so as to
thermally decompose said organic-polymer phase still remained in
said gel film and so as to transform said gel film into said porous
metal-oxide film.
3. A method according to claim 2, wherein a surface morphology of
said porous metal-oxide film is controlled by selecting said
organic polymer, by adjusting the molecular weight of and/or the
amount of said organic polymer, and/or by selecting said second
solvent.
4. A method according to claim 2, wherein said organic polymer has
a carbonyl group and thus is soluble in water or a mixed solvent of
water and alcohol.
5. A method according to claim 2, wherein said first solvent is
selected from the group consisting of alcohols and ethers.
6. A method according to claim 2, wherein said acid is selected
from the group consisting of acetic acid, hydrochloric acid and
nitric acid.
7. A method according to claim 2, wherein said second temperature
is within a range from 550.degree. to 690.degree. C.
Description
BACKGROUND OF THE INVENTION
The present invention relates to a porous metal-oxide thin film,
and a method of forming the film on a glass substrate. The
film-forming method belongs to the sol-gel process.
It is known that a porous metal-oxide thin film on a glass
substrate is effective in reducing refractive index and thus in
providing a low-reflectance glass plate. A porous metal-oxide thin
film has a very large surface area, due to its porosity. Therefore,
when it is used as a sub-layer of a multilayered film, the contact
area between the sub-layer and another layer on the sub-layer
becomes very large, and thus adhesion therebetween is much
improved. With this, the multilayered film becomes much improved in
abrasion resistance and durability.
There is known a method for preparing a porous glass such as Vycor
(a trade name of Corning Co.) by phase separation. In this method,
a sodium borosilicate glass is separated into an acid-soluble phase
rich in Na.sub.2 O.B.sub.2 O.sub.3 and an acid-insoluble phase rich
in SiO.sub.2, and then the acid-soluble phase is removed by an
acid. With this, a porous silica glass is formed. However, when
this method is used for forming a porous glass film, the film
thickness tends to become too thick.
Furthermore, there is known a method for forming a porous
metal-oxide film on a glass substrate, by etching. In this method,
at first, a metal oxide film is formed on a glass substrate. Then,
the surface of the metal oxide film is etched by hydrofluoric acid,
fluorine nitrate or the like, to make the surface porous. However,
hydrofluoric acid and fluorine nitrate are very hazardous against
human body. Therefore, these compounds must be handled very
cautiously. This lowers the production efficiency. Furthermore, the
etching step increases the production cost.
There is known another method for forming a porous metal-oxide film
on a glass substrate. In this method, at first, an organic polymer
is added to a metal alkoxide solution (sol), to prepare a coating
solution. Then, a glass substrate is coated with the coating
solution. Then, the thus coated glass substrate is heated at a
temperature not lower than the thermal decomposition temperature of
the organic polymer, to remove the organic polymer and thus to make
the metal-oxide film porous. However, during this heating, the
metal oxide film may contract very much. This leads to the
occurrence of cracks thereon. In order to prevent this, the amount
of the organic polymer may be reduced. However, as the amount is
reduced, micro-pores (micro-pits) making the film porous tend to
disappear during the contraction (densification) of the film. With
this, the surface of the metal oxide film may not become
porous.
There is still another method for forming a porous metal-oxide film
on a glass substrate, which is disclosed in U.S. Pat. No. 5,403,368
corresponding to Japanese Patent Kokai Hei 5-147976. This method
belongs to the sol-gel process. In this method, at first, a coating
solution is prepared by mixing at least two sols having different
average molecular weights with a solvent. Then, a glass substrate
is coated with the coating solution, so as to form thereon a sol
film. Then, the thus coated glass substrate is heated, so as to
transform the sol film to a porous gel film then to a porous
metal-oxide film. However, according to this method, it is
necessary to use certain special starting materials. Furthermore,
from a standpoint of pot life of the sols, it may not necessarily
easy to produce porous metal-oxide films having the same surface
morphology.
SUMMARY OF THE INVENTION
It is therefore an object of the present invention to provide a
porous metal-oxide film which is free from cracks, sufficiently
hard and superior in adhesion and weatherability.
It is another object of the present invention to provide a method
for easily and assuredly forming such porous metal-oxide film.
According to the present invention, there is provided a method of
forming on a glass substrate a porous metal-oxide film, the method
comprising the steps of:
(a) mixing together at least one compound selected from the group
consisting of metal alkoxides and metal acetylacetonates, a first
solvent, water, an acid and an organic polymer, so that a
hydrolysis and polycondensation of said at least one compound
occurs in the presence of said organic polymer and thus that a
coating solution in the form of sol having a single phase is
prepared;
(b) applying said coating solution to said glass substrate, so that
a gel film is formed on said glass substrate, said gel film having
inorganic and organic-polymer phases into which said single phase
has been separated due to an evaporation of said first solvent;
(c) drying said gel film at a first temperature so as to
substantially completely evaporate said first solvent;
(d) removing said organic polymer phase from said gel film, by
contacting said gel film with a second solvent which is one of an
acid solution and a mixed solution of alcohol and water; and
(e) heating said gel film at a second temperature so as to
thermally decompose said organic-polymer phase still remained in
said gel film and so as to transform said gel film into said porous
metal-oxide film.
According to the above-mentioned conventional method, an organic
polymer added to a coating solution is removed from a film only by
thermally decomposing the organic polymer. In contrast to this,
according to the present invention, the organic-polymer phase is
removed firstly by contacting the gel film with the second solvent
and then by thermally decomposing the organic polymer. During and
after this decomposition, micro-pits (micro-pores) and micro
roughness making the film porous do not disappear. Therefore,
according to the invention, the metal oxide film surface is
minutely rough and has a fine features pattern of three-dimensional
micro-porous structure. This fine features pattern is relatively
orderly distributed on the metal oxide film surface.
In the invention, the organic polymer in the coating solution is
chemically bonded to a part of a network of colloidal particles of
the at least one compound through esterification. However, the
organic polymer itself is soluble in the second solvent and
therefore is dissolved by the second solvent.
According to the present invention, the metal-oxide film becomes
porous even when the film has a thickness of about 500 nm. The
porous metal-oxide film of the present invention is superior in
durability, abrasion resistance and adhesion to the glass
substrate. The porous metal-oxide film can be used as a single
layer film, a multilayer film or a sub-layer of a multilayer
film.
BRIEF DESCRIPTION OF THE DRAWINGS
FIGS. 1, 2, 3, 4 and 5 are photographs taken through a scanning
electron microscope (SEM) of about 20,000 magnifications, showing
porous surfaces of metal-oxide films of Examples 2, 5 and 6
according to the present invention and of Comparative Examples 4
and 6 not according to the present invention, respectively.
DESCRIPTION OF THE PREFERRED EMBODIMENTS
According to the present invention, there is provided a porous
metal-oxide thin film, and a method of forming the film on a glass
substrate. This method includes the steps of:
(a) mixing together at least one compound selected from the group
consisting of metal alkoxides and metal acetylacetonates, a first
solvent, water, an acid and an organic polymer, so that a
hydrolysis and polycondensation of the at least one compound occurs
in the presence of the organic polymer and thus that a coating
solution in the form of sol having a single phase is prepared;
(b) applying the coating solution to the glass substrate, so that a
gel film is formed on the glass substrate, the gel film having
inorganic and organic-polymer phases into which the single phase
has been separated due to an evaporation of the first solvent;
(c) drying the gel film at a first temperature so as to
substantially completely evaporate the first solvent;
(d) removing the organic polymer phase from the gel film, by
contacting the gel film with a second solvent which is one of an
acid solution and a mixed solution of alcohol and water; and
(e) heating the gel film at a second temperature so as to thermally
decompose the organic-polymer phase still remained in the gel film
and so as to transform the gel film into the porous metal-oxide
film.
In the invention, the glass substrate is not limited to a
particular type. This glass substrate may be made of a commercial
soda lime glass. This glass substrate may be colorless or colored
as long as it is transparent. The glass substrate may be flat or
curved in shape, or a tempered one. The glass substrate may be a
laminated glass plate or a glass plate for mirror. A glass plate
having the metal-oxide film formed on the glass substrate may be
used as a low reflection glass plate, a water-repellent glass
plate, or another functional glass plate, for example, for cutting
ultraviolet rays or infrared rays.
In the invention, examples of the metal alkoxides are
tetramethoxysilane, tetraethoxysilane, methyltrimethoxysilane,
methyltriethoxysilane, titanium tetraisopropoxide, and zirconium
n-butoxide.
In the invention, an example of the metal acetylacetonates is
titanium acetylacetonate.
Examples of the first solvent are methanol, ethanol, n-propanol,
isopropanol, butanol and ethyl ether. It is preferable to use the
solvent having a higher vapor pressure. The solvent of this type
evaporates faster.
The solid matter concentration in the coating solution is
preferably within a range, on an oxide basis, from 0.01 to 0.50 wt
% and more preferably from 0.08 to 0.25 wt %. If the concentration
is less than 0.01 wt %, it may become difficult to form a film on a
glass substrate. If the concentration is more than 0.50 wt %, it
may become necessary to take the heating steps several times in
order to prevent the occurrence of cracks.
The amount of water contained in the coating solution is preferably
within a range from 4 to 10 mol to 1 mol of the at least one
compound selected from metal alkoxides and metal acetylacetonates.
If the amount of water is less than 4 mol, it may take a very long
time to dissolve the organic polymer or may fail to completely
dissolve the organic polymer. If the amount of water is more than
10 mol, the degree of gelation of the coating solution may proceeds
too much. With this, the coating solution may not be used for a
long time.
The pH of the coating solution is adjusted preferably within a
range from 1 to 3 and more preferably from 2 to 3, by the addition
of an acid to the coating solution. The amount of this acid is not
constant, but varies depending on its type. If the pH is lower than
1, the polycondensation of the metal alkoxide solution proceeds
very fast and the solubility of the organic polymer becomes small.
If the pH is higher than 3, the polycondensation of the metal
alkoxide solution proceeds very slow. Thus, it takes a very long
time to prepare the coating solution.
In the invention, the organic polymer is not limited to a
particular one, as long as it dissolves in water or a mixed solvent
of water and an alcohol and becomes uniformly mixed with the at
least one compound in the coating solution. It is preferable that
the organic polymer has a carbonyl group. Preferable examples of
the organic polymer are polyvinyl acetate, polymethyl methacrylate
and polyacrylic acid. It is preferable that the amount of the
organic polymer is within a range from 5 to 30 wt % and preferably
from 10 to 25 wt %, based on the total weight of the coating
solution. If it is less than 5 wt %, the metal oxide film may not
necessarily become desirably porous. If it is more than 30 wt %,
the residue of the organic polymer may remain in the metal oxide
film or it rnay become difficult to obtain a sufficient strength of
the metal oxide. In the invention, the organic polymer having a
molecular weight within a range from several hundreds to several
hundreds of thousands is usable. Its preferable range is from about
50,000 to about 100,000.
The hydrolysis and polycondensation of the at least one compound in
the presence of the solvent can be controlled by selecting the type
and amount of the acid as a catalyst, the pH of the coating
solution, and the reaction temperature (preferably from 20.degree.
to 40.degree. C.). This selection may vary depending on the
condition of the organic polymer at room temperature (i.e. if the
organic polymer takes liquid form or solid form) and/or the type of
the at least one compound. It is preferable that the at least one
compound is added to the solution in which the organic polymer has
already dissolved in the solvent.
The surface morphology of the metal oxide film and the diameter of
the micro-pits can be controlled by selecting the amount of the
organic polymer and the type of the second solvent for removing the
organic polymer.
It is preferable that the drying temperature at the step (c) is
lower than 100.degree. C. and more preferably within a range from
about 40.degree. to about 90.degree. C. If this temperature exceeds
100.degree. C., the inorganic elements contained in the sol are
concentrated as the solvent evaporates. Thus, the film structure
becomes gradually rigid or hard. With this, the efficiency for
removing the organic polymer may be lowered, resulting in the
occurrence of cracks in the metal oxide film.
The heating temperature at the step (e) is preferably a temperature
at which the glass substrate is not damaged, at which the porous
morphology of the metal oxide film does not disappear, at which the
metal oxide film is sufficiently improved in hardness, and at which
the organic polymer remained in the film is completely removed.
Thus, the heating temperature is preferably higher than the
decomposition (combustion) temperature of the organic polymer by at
least about 100.degree. C. It is more preferably within a range
from about 550.degree. to 690.degree. C., within which the heating
step can be conducted while the bending process or the tempering
process is conducted on the glass plate. It is still more
preferably within a range from about 570.degree. to 670.degree.
C.
The porous metal oxide film formed on the glass substrate according
to the invention does not have defects such as cracks and the like
nor damage the glass substrate in optical characteristics, and is
sufficient in mechanical strength and superior in adhesion to the
glass substrate, abrasion resistance and durability.
The porous morphology (i.e. the minute roughness and the presence
of micro-pores) of the metal oxide film is formed by the phase
separation immediately after the gelation of the film applied to
the glass substrate, and then by the removal of the organic polymer
phase with the second solvent. This removal occurs because the
organic polymer dissolves in the second solvent by a certain
degree. The micro-pores do not disappear even after the heating at
the step (e).
According to the present invention, the organic polymer remained in
the film is removed by decomposition (combustion) thereof.
Furthermore, at the step (d), it is possible to remove not only the
high-polymer phase but also a portion of the inorganic phase. The
porous morphology formed by the step (d) does not substantially
change by the step (e). Furthermore, compressive stress caused by
the heating step (e) is diminished, because the film has a
so-called soft structure formed by removing the polymer phase at
the step (d). With this, it becomes possible to suppress the
occurrence of cracks. Thus, it is possible to add a relatively
large amount of the organic polymer to the coating solution.
As is mentioned hereinabove, the organic polymer is soluble in
water or a solvent mixture of water and an alcohol. The inorganic
structure in the gel film after the drying is not so developed. In
the invention, the sol is prepared in the presence of the organic
polymer, and thus the polycondensation (the development of the
inorganic structure) does not proceed so much, as compared with a
case in which a sol is prepared not in the presence of the organic
polymer. Therefore, it becomes possible to remove the organic
polymer phase and a portion of the inorganic phase.
In the invention, the porous metal-oxide film is almost free of the
organic polymer and its residue and free of cracks even when the
film thickness is within a range from 500 to 630 nm.
The following nonlimitative Examples 1 to 6 are in accordance with
the present invention, and the following Comparative Examples 1 to
6 are not in accordance with the present invention.
Example 1
In this example, a coating solution was prepared as follows. At
first, 10 parts by mol of a special-grade ethyl alcohol made by
Pharmco Co. was mixed with 8 parts by mol of water containing about
0.05 mol % of HCl so as to prepare a first mixed solution. Then,
about 10 wt %, based on the total weight of the coating solution,
of a polyvinyl acetate (PVAc) in the form of granule which has an
average molecular weight of about 83,000 and is made of Aldrich Co.
was completely dissolved in the first mixed solution at room
temperature so as to prepare a second mixed solution. Then, 1 part
by mol of a tetraethylorthosilicate (TEOS) made by Aldrich Co. was
added to the second mixed solution, followed by stirring at room
temperature for about 24 hr, so as to prepare the coating solution.
This coating solution (sol) had a pH of about 2.0 and a viscosity
of about 15 cP.
Separately, a slide glass plate made by Corning Co. and having a
product No. of 2947, a size of about 3 inch.times.about 3 inch and
a thickness of about 1 mm was washed with a neutral detergent, then
with water and then with alcohol, then dried, and then wiped with
acetone. Then, this slide glass plate was immersed into the
above-prepared coating solution, and then withdrawn therefrom at a
rate within a range from 2 to 5 mm/sec, so as to prepare a gel film
on the slide glass plate. Then, the slide glass plate having
thereon a gel film was dried at about 80.degree. C. for about 10
min. Then, this slide glass plate was immersed in a mixed solvent
containing 1 part by volume of ethyl alcohol and 1 part by volume
of water for about 5 min. Then, this slide glass plate was
withdrawn therefrom and dried again at about 80.degree. C. for
about 10 min. Then, the slide glass plate was heated in an electric
furnace for about 30 min with a temperature increasing rate of
about 10.degree. C./min, so as to obtain a SiO.sub.2 -film-coated
slide glass plate.
The SiO.sub.2 film on the slide glass plate was evaluated by the
following tests. The film thickness was optically determined with a
spectrophotometer (Lambda 9 UV-VIS-NIR made by Perkin Elmer Co.).
The surface condition was observed with a SEM (Amray, 1400,
Bedford, Mass.) with magnifications of 20,000 and 30,000 and
acceleration voltages of 30 Kv and 15 Kv. In this observation, the
surface of the SiO.sub.2 film was examined whether or not defects
such as cracks are formed on the surface. The average diameter of
micro-pores was determined by a N2-gas adsorption and desorption
method with Omnisorp 360 (trade name) made by Omicron Technology
Co., Berkley Heights, N.J., and a degassing at 150.degree. C. for
24 hr under vacuum condition. The results are shown in Table.
Vickers hardness of the SiO.sub.2 film was determined with Zwick
3212.00 tester with an impact speed of 0.3 mm/sec, an indent time
of 15.0 sec and a load of 0.2 kg, and the result was about 8 kg/mm.
The SiO.sub.2 film was superior in adhesion, abrasion resistance
and durability.
By the above tests, it was found that the SiO.sub.2 film surface is
minutely rough and has a fine features pattern of three-dimensional
micro-porous structure. This fine features pattern was relatively
orderly distributed on the SiO.sub.2 film surface.
Example 2
In this example, Example 1 was repeated except in that the amount
of the PVAc was increased to about 20 wt % based on the total
weight of the coating solution. The coating solution had a
viscosity of about 35 cP and a pH of about 2.0. The test results
are shown in Table. The SiO.sub.2 film had the fine features
pattern as described in Example 1, and was superior in adhesion,
abrasion resistance and durability.
Example 3
In this example, Example 1 was repeated except in that the amount
of the PVAc was increased to about 25 wt % based on the total
weight of the coating solution. The coating solution had a
viscosity of about 50 cP and a pH of about 2.0. The test results
are shown in Table. The SiO.sub.2 film had the fine features
pattern as described in Example 1, and was superior in adhesion,
abrasion resistance and durability.
Example 4
In this example, Example 1 was repeated except in that the slide
glass plate coated with the dried gel film was immersed for about 5
min in a nitric-acid aqueous solution containing about 0.01 mol %
of nitric acid, in place of the mixed solvent of ethyl alcohol and
water. The coating solution had a viscosity of about 15 cP and a pH
of about 2.0. The test results are shown in Table. The SiO.sub.2
film had the fine features pattern as described in Example 1, and
was superior in adhesion, abrasion resistance and durability.
Example 5
In this example, Example 2 was repeated except in that the amount
of the PVAc was increased to about 20 wt % based on the total
weight of the coating solution. The coating solution had a
viscosity of about 35 cP and a pH of about 2.0. The test results
are shown in Table. The SiO.sub.2 film had the fine features
pattern as described in Example 1, and was superior in adhesion,
abrasion resistance and durability.
Example 6
In this example, Example 2 was repeated except in that the amount
of the PVAc was increased to about 25 wt % based on the total
weight of the coating solution. The coating solution had a
viscosity of about 50 cP and a pH of about 2.0. The test results
are shown in Table. The SiO.sub.2 film had the fine features
pattern as described in Example 1, and was superior in adhesion,
abrasion resistance and durability.
Comparative Examples 1-3
In these comparative examples, Examples 1-3 were respectively
repeated except in that the first and second drying processes and
the immersion process for removing the polymer phase were omitted.
In fact, the coated slide glass plate was heated at about
600.degree. C. for about 30 min with a temperature increasing rate
of about 2.degree. C./min. The test results are shown in Table. In
each of these comparative examples, the SiO.sub.2 film had many
fine cracks, was in a condition that determination of the
micro-pore diameter is impossible, and did not have the fine
features pattern as found in Examples 1-6.
Comparative Examples 4
In this comparative example, Example 1 was repeated except in that
the immersion process for removing the polymer phase was omitted
and that the coating solution was prepared as follows.
At first, a first solution was prepared by mixing together 40 ml of
TEOS made by Aldrich Co., 40 ml of ethanol made by Pharmco Co., and
15 ml of water containing 0.1 mol % of HCl, followed by a further
mixing through reflux at about 60.degree. C. for about 7 hr.
Separately, a second solution was prepared by mixing together 40 ml
of methyltrimethoxysilane made by Aldrich Co., 40 ml of ethanol
made by Pharmco Co. and 15 ml of water containing about 0.1 mol %
of HCl, followed by a further mixing through reflux at room
temperature for about 7 hr.
The first and second solutions were mixed together in a volumetric
ratio of 3.5 to 1.5, and then this mixture was stirred at room
temperature for not less than about 24 hr, so as to prepare the
coating solution. The coating solution had a viscosity of about 3
cP and a pH of about 4.0. The SiO.sub.2 film did not have the fine
features pattern as found in. Examples 1-6.
Comparative Examples 5
In this comparative example, Example 1 was repeated except in that
the immersion process for removing the polymer phase was omitted
and that the coating solution was prepared as follows.
At first, 1 part by mol of TEOS made by Aldrich Co. was mixed with
2 part by mol of ethanol made by Pharmco Co. so as to prepare a
first solution. Separately, a polyethylene glycol (PEG) in an
amount to have a molar ratio of PEG to TEOS of 0.25:1 on a monomer
(--CH.sub.2 CH.sub.2 O--) basis was mixed with hot water, so as to
prepare a second solution. This PEG had a molecular weight within a
range from 570 to 630 and an average molecular weight of 600, and
was made by Aldrich Co. The first and second solutions were mixed
together, and then sufficiently stirred at room temperature in the
air for at least about 24 hr, so as to prepare the coating
solution. The coating solution had a viscosity of about 10 cP and a
pH of about 1.5. The SiO.sub.2 film did not have the fine features
pattern as found in Examples 1-6.
Comparative Examples 6
In this comparative example, Comparative Example 5 was repeated
except in that the PEG was in an amount to have a molar ratio of
PEG to TEOS of 0.5:1 on a monomer (--CH.sub.2 CH.sub.2 O--) basis.
The coating solution had a viscosity of about 18 cP and a pH of
about 1.5. The SiO.sub.2 film did not have the fine features
pattern as found in Examples 1-6.
TABLE ______________________________________ Film Thickness
Occurrence of Average Pore (nm) Cracks Diameter (nm)
______________________________________ Example 1 250 no cracks 30
Example 2 260 no cracks 80 Example 3 240 no cracks 100 Example 4
230 no cracks 60 Example 5 260 no cracks 100 Example 6 235 no
cracks 130 Com. Ex. 1 250 a large number N/A of fine cracks Com.
Ex. 2 240 a large number N/A of fine cracks Com. Ex. 3 270 a large
number N/A of fine cracks Com. Ex. 4 200 no cracks 15 Com. Ex. 5
240 no cracks 25 Com. Ex. 6 260 a few cracks 40
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